"Unauthorized Access. Session May have expired" Error.

Hi, Our customers are periodically getting the following error while trying to publish an approval: "Unauthorized Access We're sorry. Your session may have expired. Please try again. If you receive this message multiple times, please contact support." I just checked the last customer who got the error and the user is listed as a sender, so that's not the issue. Can you help me figure out in what scenarios this error would occur? Thanks, Fernando